The present invention relates generally to shaft position recorder apparatus and method, and in particular, to such recorder apparatus and method employing an electronic memory to store a difference signal corresponding to a change in shaft position which is periodically gated into such memory if such difference signal exceeds a predetermined amount or "deadband". The shaft position recorder of the present invention is especially useful for recording the change in liquid levels, such as water levels of rivers, and indicating whether the change in water level is up or down from the previous measured level. The recorder includes a shaft position encoder to produce a group of electrical pulses for each measured change in shaft position or liquid level with the sequence of the pulses in each group indicating whether the change was an increase or decrease from the previous reading. As a result of recording only difference signals exceeding a predetermined deadband, the recorder of the present invention operates more efficiently so it can employ a memory of small storage capacity to accurately record the changes in shaft position and the direction of such change. In addition, the recorder includes a maximum memory and a minimum memory which continuously monitor the shaft position signal and temporarily store the maximum and minimum position signal which are gated to the permanent memory at the end of a predetermined time period, such as 24 hours.
Previously it has been proposed in U.S. Pat. No. 2,082,038 of West et al, to provide a liquid level recording system which transmits an electrical signal corresponding to the liquid level from a remote location to a relay operated level indicator using a shaft position encoder and float to produce such signal. The shaft encoder is in the form of a cam wheel having two sets of cams which operate a pair of switches to cause them to transmit pulses of different polarity to the recorder to indicate a rise or fall of the liquid level by the sequence of such pulses. Unlike the recorder of the present invention, this prior apparatus does not employ a periodically gated memory for storing a difference signal corresponding to the difference between successively measured liquid levels. Also it does not employ a memory for recording the maximum and minimum levels produced during a predetermined time period. Further, the shaft encoder of West differs from that of the present invention in that the switches are not magnetically actuated by a polarity of magnets and does not employ three switches which are operated in different sequence to indicate a rise or fall of the liquid level.
Previously it has been suggested in U.S. Pat. No. 4,008,405 of Neumann et al, to provide an electronic weighing system including a motion detector which detects motion and inhibits the production of an output weight signal when such motion exceeds predetermined limits. This motion detector compares the analog input signal at the output of an integrator with the stored prior input signal to produce a difference signal which is compared with reference signals to produce a motion signal which inhibits the output weight signal when the difference signal exceeds the reference signals. Unlike the present invention there is no difference output signal corresponding to a change in liquid level or shaft position which is periodically recorded in a gated permanent memory. Also the reference signals are not stored in a datum memory which is updated after each reading. Furthermore, there is no shaft position encoder for producing pulses and pulse analyzer to determine from the sequence of such pulses whether the change in level is up or down from the previous level in the manner of the present invention.
Also it has previously been proposed in U.S. Pat. No. 2,922,994 of Kennedy to provide a shaft position encoder signal generator employing a polarity of magnetically actuated reed switches which are operated by magnets attached to a support rotated by a shaft. However, this encoder is a high-speed analog to digital converter and is not used as the input of a liquid level recorder to indicate the direction of change in level by the sequence of the pulses in the manner of the present invention.